1. Field of the Invention
The present invention is related to wireless communication systems. More particularly, the present invention is directed to a method of supporting voice band modem-to-modem calls in a wireless communication system.
2. Related Art
Telecommunication providers often employ wireless communication technology as a cost-effective means of connecting a customer/end-user location (such as a residence or business) to the provider's network (historically the local voice switch). In the past, this connection typically consisted of a twisted-pair copper wire that supported voice-band services, such as voice calls, fax calls, and voice-band modem calls. These voice-band services are typically carried on circuit-switched channels. When using wireless communications technology as a replacement for or an alternative to copper wire, it is desirable for the wireless link to support the same set of voice-band services as the copper link as well as high bandwidth broadband data services. High bandwidth broadband data services are typically carried on packet-switched channels.
Due to bandwidth constraints caused by limited wireless spectrum availability and/or the extremely high cost of acquiring spectrum rights, wireless communication systems that provide voice access typically utilize voice coding to reduce the bandwidth required by a voice call. Researchers have made significant strides in recent years that allow systems to provide high-quality voice transmissions at very low data rates. A difficult dilemma, however, exists for the creators of wireless technology: How does one enable high-rate voice-band modems, with speeds of up to 56 kilobits per second (kbps), to use a system designed for low-rate voice transmissions, with speeds that may be below 16 kbps?
One solution is to use a tone detector to determine when a voice-band modem session is active. When a modem call is detected, multiple low-rate voice channels are combined together and assigned to that call, such that sufficient bandwidth is provided to support the higher-rate modems. The number of channels that are combined depends upon the modulation standard of the modems and the voice coding of the system. For example, if the modems are using the V.34bis standard with transmission speeds of up to 33.6 kbps and the system employs a 16 kbps voice coder and has 16 kbps wireless voice channels, a minimum of three channels must be combined for there to be sufficient bandwidth to support a modem call. Oftentimes, lower rate voice channels are combined into a 64 kbps channel that carries μ-law or A-law encoded Pulse Code Modulation (PCM) data to support voice-band modem services.
There are, however, several drawbacks to the above approach. One drawback is that the combined channels are dedicated (i.e., the assigned channels are not used for other communications) for the duration of the modem call. This results in a reduction in the total system capacity with every active modem call. Another drawback is the fact that a significant portion of the bandwidth of the combined channels is often times wasted. For instance, in the example above, the modem call may only utilize 33.6 kbps of the 48 kbps-bandwidth of the combined three voice channels, which is a waste of 14.4 kbps or 30% of the bandwidth.
In addition, data traffic is inherently ‘bursty’. For example, if a user has dialed into a modem server and is browsing the Internet, data transfers only take place after the user clicks on a web link. After a web page has been downloaded, other data transfers usually do not take place. With the above solution, however, the voice channels are still assigned to the call. This results in further waste of airlink resources. Thus, there is a need for a wireless communication system that supports both low-rate voice channels and high-rate voice-band modem access without changing voice capacity regardless of the number of active voice-band modem calls.